The present invention relates to a speed variator pulley provided with removable rings having a reduced friction and used, namely, in a variable speed transmission of the "V" belt type pulleys and trapezoidal belt. Transmissions of this type are, for example, used in snow scooters, motorcycles, utilitarian or pleasure vehicles or cars without permit or of the TQM type.
They generally comprise a driven pulley solid with a drive shaft and a driving pulley drawn into rotation by a belt lying in the grooves of the two pulleys. The driving pulley is adapted so the winding diameter of the belt that is automatically adjusted as a function of the speed of rotation of the pulley. The belt having a set length, the driving pulley has a winding diameter which varies inversely of the driving pulley. Under the combined effect of these two variations, the ratio of the speeds of rotation of the two pulleys is modified according to the changes in diameter of the driving pulley.
As examples of driving pulleys of the type mentioned previously as described in FR-A-2,504,635 and CA-A-1,212,559.
They generally comprise two coaxial cheeks having frustoconical sidewalls facing one another and forming between them a V-shaped groove. A cheek, called the fixed or fast cheek, is secured in rotation and in translation on the drive shaft of the pulley and the other, called the movable cheek, may slide axially on the shaft. Behind the movable cheek, there is provided an oblique ramp of which the surface turned toward the axis of the shaft faces a second oblique ramp of a reaction cup solid with the drive shaft of the pulley. Inertia weights provided in housings of the movable cheek and in housings of a reaction cup are capable of radial displacement and are guided, in this movement, by the lateral walls of these housings.
When the shaft of the pulley rotates, it drives the fixed cheek and the reaction cup into rotation. The latter transmits its rotation motion, through the lateral walls of the housings of the reaction cup, to the inertia weights which in turn drive the movable cheek through the lateral walls of the housings of said movable cheek, On the other hand, because they rotate, the inertia weights are subjected to the action of centrifugal forces and, moving radially, respectively bear against the two ramps of the movable cheek and those of the reaction. Because of their trapezoidal shape, they exert an axial push on the movable cheek which then slides toward the fixed cheek. For a given rotation speed of the pulley, an equilibrium is reached when this axial push is counterbalanced by an axial return force exerted by return springs mounted between the movable cheek and a shoulder of the shaft of the pulley.
According to this equilibrium, the winding diameter of the belt of trapezoidal cross section takes on a value which varies depending on the speed of rotation of the pulley.
In dynamics, when the speed of rotation increases, the centrifugal forces on the inertia weights increase proportionally. The axial force of the latter on the ramps increases equally and the equilibrium is displaced toward close-up of the two cheeks. Therefore, the winding diameter of the belt increases. Inversely, when the speed decreases, the axial thrust of the inertia weights decreases, and the equilibrium is displaced toward distancing of the two cheeks. The winding diameter of the belt thereby decreases.
In fact, axial forces generated on various points of the pulley oppose the axial movement of the cheek movable on the drive shaft. They usually originate from frictional forces with may be generated, for instance, by contact of the inertia weights with the ramps, by contact of the movable cheek with the drive shaft, etc. These axial forces generate a hysteresis effect in the operation of the pulley. Indeed, when the speed increases, the movable cheek is displaced toward the fixed cheek. The axial reaction forces oppose this motion of the movable cheek and avoid the axial thrust exerted by the inertia weights. The equilibrium is thus displaced toward a lesser closing in of the two cheeks and the winding diameter is consequently smaller than what it should be without these frictions forces. On the other hand, when the speed of rotation decreases, the movable cheek moves away from the fixed cheek. The axial friction forces which oppose this motion thereby tend, in this case, to hold the two cheeks close to one another. The winding diameter is thereby greater than what it should be without these friction forces.
Among the axial forces which originate from the friction forces, those that are generated by the contact of the movable cheek with the drive shaft are of interest here. They have values proportional to the radial forces applied on the guide shaft by the movable cheek. Now, these radial forces originate from the reactions generated by the belt on the movable cheek. Under the influence of these forces of which the radial resultant is not zero, the movable cheek has a tendancy, when rotating, to tilt with respect to an axis normal to the axis of the pulley. These reactions forces in fact form a torque which is compensated by the one which is formed, with respect to the same normal axis, by the reaction of the movable cheek on the drive shaft at their points of contact located symmetrically on either side of the said normal axis.
In order to lessen the hysteresis effect in the operation of the pulley, it is necessary to provide means which allow to reduce the axial friction forces developing between the movable cheek and the drive shaft.
It will be noted here that the transmission of which the driving pulley has too large a hysteresis effect reduces the performances of vehicles having such a transmission.
Three essential problems are to be solved to reduce in an efficient manner the friction forces between the movable cheek and the drive shaft. The first one lies in providing slide bearings for the movable cheek which have to be of the type having a small coefficient of friction but which also have to be easy to install and to replace, which is not generally the case since such bearings are force fitted and necessitate tooling for their mounting and removal.
The second one lies in providing an arrangement allowing to increase the distance between the two terminal bearing points of the movable cheek on the drive shaft but which also allows the mounting of a disconnection ring on the said shaft at the bottom of the groove of the pulley; the mounting of this ring generally necessitating that this length be reduced.
The third problem is to provide an arrangement which is appropriate for solving the previous problems and which further allows to shorten the distance comprised between the point of contact of the belt with the movable cheek and the pivot axis of the movable cheek. By decreasing this length, the reaction torque of the cheek on the shaft is decreased and, consequently, the axial and radial forces generated by this torque.